{n. phr.}, {informal} Something difficult to understand or to do. * /Tom's algebra lesson was a hard nut to crack./ * /Mary found knitting a hard nut to crack./ Compare: HARD ROW TO HOE.
TOUGH NUT TO CRACK
Meaning of TOUGH NUT TO CRACK in English
Slang English vocab. Английский сленговый словарь. 2012